In May, the supermassive black hole at the core of the Milky Way became 75 times brighter in just two hours.

The supermassive black hole that lives at the center of our galaxy has been mysteriously sparkling as of late, and nobody knows the reason.

This dark behemoth, known as as Sagittarius A* (Sgr A*), is four million times as massive as the Sun. Though no light escapes its boundaries, astronomers can observe the hole’s interactions with bright stars or dust clouds that surround it.

On the night of May 13, 2019, UCLA astronomer Tuan Do and his colleagues were watching Sgr A* using the Keck Telescope on the summit of Mauna Kea in Hawai’i. In a period of just two hours, they witnessed the black hole become 75 times brighter in the near-infrared band of the light spectrum.

That spring evening, the Milky Way’s supermassive black hole “reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths,” according to a forthcoming study, led by Do and published on the arXiv preprint server.

“The brightness of Sgr A* varies all the time, getting brighter and fainter on the timescale of minutes to hours—it basically flickers like a candle,” Do said in an email. “We think that something unusual might be happening this year because the black hole seems to vary in brightness more, reaching brighter levels than we've ever seen in the past.”

The peak flux, meaning the most luminous phase of the flare-up, soared to “twice the maximum historical flux measurements,” Do’s team said in the study. In other words, in the 20 years since astronomers have monitored Sgr A*, the next-brightest event has only been half as dazzling as this one.

This unusual sparkle at the galactic core was likely caused by close encounters between Sgr A* and objects surrounding it, according to the team.

The edge of a black hole, called an event horizon, is shaped by intense tidal forces that tear at anything that gets close. Once a black hole starts devouring nearby objects like stars or gas clouds, infalling material heats up at the event horizon, sparking light shows that can be picked up by telescopes.

Do and his colleagues speculate that a star called S0-2, which is about 15 times as massive as the Sun, may have been the object that juiced Sgr A*. In 2018, S0-2 came within 17 light hours of the supermassive black hole, and that close pass may have disturbed gases at the event horizon enough to cause the May 2019 brightening event.

That initial letdown may have been premature, though, because we might be seeing the slow-burn “delayed reaction” to the gas cloud’s approach, the team said.

“Many astronomers are observing Sgr A* this summer,” Do noted. “I'm hoping we can get as much data as we can this year before the region of the sky with Sgr A* gets behind the Sun and we won't be able to observe it again until next year.”

“Maybe the black hole is waking up—there's a lot we don't know at this point so we need more data to understand if what we are seeing is a big change in what is feeding the black hole or this is a brief event,” he said.